If both www and non www domain of your website (http://example.com and http://www.example.com) serve the same website, and you want people going to http://example.com to automatically be redirected to http://www.example.com, there's a few way that you can do so with Apache

First of all, make sure mod_rewrite is installed and enabled for your Apache installation. Once it is, add a file named .htaccess in your website's root directory and add the following lines;

RewriteEngine On
RewriteCond %{HTTP_HOST} ^!example.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]

You can also change your website's Virtualhost setting for the redirect.

<VirtualHost *:80>
    ServerName example.com
    Redirect permanent / http://www.example.com/
</VirtualHost>

Restart Apache for the changes to take effect